Honestly, I don't see why background processing en webservice calls might be in contradiction.
What we have to do: we have an event-process for thousands of students and we need to send these data to each agenda in Exchange.
We want to use the load distribution and the queue of bgRFC to send all individual data to Exchange.
This is done with a synchronous webservice that returns a GUID we store in an SAP table. The webservice is a service we can't change. And the returning GUID we need for later updates (or deletes) of the agenda-item.
We are aware of all possible risks this architecture might cause, but I don't see why it should not be technically possible.
SAP says it is too time consuming to look for a possible error ...